1. Field of the Invention
The present invention generally relates to a spinal fixation rod made of titanium alloy rod for coupling with and fixing screws used for fixing the spine.
2. Background Art
Typically, a spinal fixation appliance is utilized as a system assembled from various types of parts and is prepared with a size and shape according to the intended use. A typical example suitable as a spinal fixation appliance is a spinal fixation system for fixation of the vertebrae by fixing a plurality of screws embedded in the vertebrae.
The spinal fixation rod is normally shaped as a 4 to 7 mm diameter cylinder having sufficient length for coupling with and fixing the plurality of screws embedded in the vertebrae. During an operation, while the physician elastically deforms the rod to match the curvature of the vertebrae of the spine of the human body, the rod for coupling and fixing the plurality of screws embedded in the vertebrae is coupled to a receiving part of the head part of the screws, and then a rod approximetor is used to fix the rod to the screw heads using set screws.
Providing the rod with biocompatibility is important, because the rod is used by embedding the rod for a long time within the human body. In addition to the rod needs to be capable of being bent by the physician in order to be deformed during the operation to match curvature of the vertebrae of the spine, the rod also preferably does not lose toughness at the bent part. Conversely, a spinal fixation rod embedded in the human body may be subjected to large loads from various angles due to bending and stretching of the vertebrae associated with movement of the human body. Therefore, the rod is required to resist elastic deformation due to a load. However, stress shielding occurs when the spinal fixation appliance is excessively hard in comparison to the bones of the human body. Therefore, the rod preferably has an elastic modulus similar to that of human bone. The rod absolutely must have a fatigue strength of at least a certain value in order to be used long term within the human body. In order for the rod to be capable of use as a spinal fixation rod, the material constituting the rod is required to simultaneously satisfy such mutually contradictory requirements as described above.
Although stainless steel, Co—Cr alloy, pure titanium, and Ti alloy are known as materials for use in a spinal fixation rod, finding a material that satisfies, with good balance, the above-described severe conditions required for a spinal fixation rod is not easy. Among the above-described materials, Ti alloys have recently been frequently used as materials for spinal fixation rods because of their extremely high safety due to resistance to corrosion in the human body and good compatibility with tissues.
Ti alloy spinal fixation rods have been manufactured heretofore as appliances made from Ti-6Al-4V (mass %) alloy, and such spinal fixation rods are used most often due to resistance to elastic deformation due to load (permanent deformation) or the like. However, such Ti alloy spinal fixation rods have been indicated to have problems such as the ready occurrence of stress shielding, because the Ti-6Al-4V alloy has a high elastic modulus in comparison to human bone, vanadium capable of harming the human body is included, and the like.
Under such circumstances, a spinal fixation rod and a spinal fixation system using such a spinal fixation rod are presently needed in order to overcome all the deficiencies of the spinal fixation rods using the Ti-6Al-4V alloy. The spinal fixation rod and spinal fixation system should have characteristics meeting, with good balance, the various requirements for a spinal fixation rod during a spinal fixation operation.